Gregorian antenna with ring focus

ABSTRACT

A Gregorian antenna system has the usual main reflector, subreflector, a first feed for illuminating the main reflector from the virtual focus via the subreflector, and a second feed for directly illuminating the main reflector from a prime focus. The second feed is shaped, and is positioned relative to the subreflector, to create a ring focus for energy reflected from the subreflector to the main reflector, and the main reflector is shaped to have the ring focus as its prime focus.

United States Patent [191 Sciambi, Jr.

in] 3,710,341 51 Jan. 9, 1973 [22] Filed:

[54] GREGORIAN ANTENNA WITH RING FOCUS [75] Inventor: Attilio F. Sciambi, .Ir., Melbourne,

Fla. 32935 [73] Assignee: Radiation lnc., Melbourne, Fla.

March 17, 1971 [21] App]. No.: 125,265

[52] U.S. Cl. ..343/779, 343/782, 343/837, 343/840 [51] Int. Cl. ..H0lq 19/14 [58] Field of Search ..343/781, 837, 840, 779, 782

[56] References Cited UNITED STATES PATENTS 3,276,022 9/1966 Brunner ..343/840 l/1966 Matson et a! ..343/837 3,500,419 3/1970 Leitner et a1. ..343/840 Primary Examiner-Eli Lieberman Attorney-Donald R. Greene [57] ABSTRACT A Gregorian antenna system has the usual main reflector, subreflector, a first feed for illuminating the main reflector from the virtual focus via the subreflector, and a second feed for directly illuminating the main reflector from a prime focus. The second feed is shaped, and is positioned relative to the subreflector, to create a ring focus for energy reflected from the subreflector to the main reflector, and the main reflector is shaped to have the ring focus as its prime focus.

9 Claims, 2 Drawing [Figures 1. Field of the Invention The present invention resides in the field of reflectortype antennas and is particularly directed to a Gregorian antenna system which is suitable for simultaneous use at two different frequencies and for two or more functions.

2. Prior Art As is well known, the Gregorian antenna system util izes a pair of confronting curved reflectors or dishes, the larger or main reflector of which is a parabolic reflector, i.e., a paraboloid constituting the surface of revolution generated by rotating the arc of a parabola about the line joining the vertex and the focal point thereof. The'parabolic reflector has a prime focus, from which it is illuminated by radio frequency (RF) energy emanating from one feed, and a virtual focus from which it is illuminated via subreflector by RF energy supplied by a second feed. The second feed has its phase center positioned coincident with the virtual focus and directly illuminates the sub-reflector, or second reflector of the Gregorian system. The subreflector is elliptical in shape, i.e., is an ellipsoid constituting a surface of revolution generated by rotating the arc of an ellipse about the line joining its vertex and its focal point, coincident with the prime focus of the parabolic reflector. The subreflector is constructed and arranged to reflect energy incident thereon toward and onto the surface of the parabolic reflector which in turn collim ates it and directs it toward a target and/or a receiving or transmitting station.

The position of each feed necessarily causes some aperture blockage, as well as interference between the two signals with which the feeds are associated. It is customary practice in the design of a reflector antenna system to attempt to reduce the amount of such blockage and interference as much as is practicable.

An example of this practice is illustrated by the dis-closure of US. Pat. No. 3,438,041, granted Apr. 8, 1969,

SUMMARY OF THE INVENTION It is a principal object of the present invention to provide a Gregorian antenna system in which the feed whose phase center is at the prime focal point 'to directly illuminate the parabolic reflector is furtherintentionally constructedand arranged to obstruct and reflect energy associated with the elliptical reflector to create a secondary focus relative to the prime focal point, in the form. ofa ring focus.

. Briefly, according to an embodiment of the present invention, the feed at the prime focus of the parabolic reflector, and typically supported at or near the vertex of the elliptical reflector, is provided with an external cylindrical configuration or is covered by a cylindrical housing, or is otherwise configuredor covered, to obstruct at least a substantial portion of the energy reflected from the ellipsoid from passing directly through the prime focal point toward the parabolic reflector. When the individual. rays of RF energy impinge upon the reflective cylindrical surface of the feed, they are redirected toward the parabolic reflector through a ring focus about the prime focal point and having its center on the line joining the prime focal I point and the vertex of the parabolic reflector. That is I to say, the ring focus is attributable to the optical properties of the surface or housing of the feed in conjunction with those of the elliptical reflector. Since the ring focus is displaced from the prime focal point by an amount A equal to the radius of the ring, the parabolic reflector must be appropriately shaped such that its prime focus conforms to such a ring. This is achieved using a figure of revolution based on a curve (a flattened parabola) having a locus of points defined by the equation (yA) 4 fx, where f is the focal length of the parabola, x and y are coordinates in the Cartesian coordinate system, and A is as previously defined. Upon revolution of this curve about its axis, a flattened paraboloidal surface is generated.

The present invention thereby takes advantage of the presence of the feed that directly illuminates the main reflector at a position relative to the subreflector which normally produces some scattering of energy and signal interference, to appropriately provide that feed with optical properties which, in conjunction with the optical properties of the main and subreflectors, can assure that virtually all of the energy from the feeds will be transmitted without interference. Thus, an extremely high gain, highly efficient antenna system is realized in eliminated by providing the exterior surface of that.

feed with a shape suitable for producing a ring focus about the usual prime focus of the main parabolic reflector and by appropriately shaping the surface con tour of the main reflector as a flattened paraboloid having the ring focus as its prime focus.

BRIEF DESCRIPTION OF THE DRAWINGS In describing the construction and function of an embodiment of the present invention, reference will be madeto the accompanying figures of drawing, in which FIG. 1 is a fragmentary sectional schematic view of a Gregorian antenna system utilizing the principles of the invention; and

FIG. 2 is a more detailed fragmentary sectional schematic view of the reflecting feed and associated subreflector, illustrating the path ofrays incident thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to FIG. 1, a basic Gregorian antenna system includes a main reflector 11 having a highly reflective surface on its concave contour. A first feed 12 in the form of a waveguide 13 terminating in a horn 14 is supported at its point of entry through the vertex of main reflector 11 by a suitable support structure 15. In this regard, it will be observed that the figures of drawing and the accompanying description do not include details of such items as support structures, fasteners, and the like, which are not essential to an understanding of the present invention and which are well known in the antenna field.

The phase center of feed 12 is located at the virtual focal point A of main reflector 11, as determined by the position of an elliptical reflector 18, also having a highly reflective surface coating on its concave contour. Elliptical reflector or ellipsoid 18 serves as a subreflector in the Gregorian antenna system, and would share a common prime focal point B with main reflector 11 if the latter were purely paraboloidal. The vertices of the two reflectors 11, 18 lie along a common axis containing focal points A and B. A second feed 19, of any suitable type and having the prime focal point B as the phase center of RF energyemanating therefrom, projects through an opening in the vertex of subreflector 18 and is supported at the position by a suitable support member 20.

The first feed, 12, illuminates the subreflector 18 and the RF energy thus impinging on that reflector is directed back toward the prime focus B for passage therethrough and illumination of main reflector 11. The main reflector collimates these rays" of RF energy and directs them toward a target or a remote station.

According to the present invention, the second feed, 19, is of any appropriate type according to the frequency of RF energy to be transmitted therefrom, but the external surface of the feed is cylindrical in shape or, alternatively, the feed is covered by a cylindrical housing throughout except at its mouth. As a result of this configuration, and the location of the prime focal point B at or near the mouth of feed 19, the rays of RF energy striking the subreflector 18 and which would otherwise pass through the prime focal point 13 instead impinge upon and are reflected from the cylindrical external surface of feed 19. The external cylindrical surface of feed 19 is made highly reflective and since the angle of reflection is equal to the angle of incidence, the rays striking that surface are reflected toward main reflector 11 in a symmetrical array about the axis of the Gregorian system, as shown in FIG. 2. The effect is to create a ring focus C about the axis, each of the rays passing through the ring focus in its path toward the main reflector 11. This requires that main reflector ll be appropriately shaped to possess such a ring focus as its prime focus.

To provide such shaping, the paraboloid which would ordinarily constitute the main reflector in a Gregorian antenna system must be symmetrically displaced or offset from the axis of the system by a distance, A equal to that distance by which the ring focus is symmetrically displaced or offset from prime focal point B. The distance A is, of course, simply the radius of ring focus C. Mathematically, the main reflector isdefined as a figure of revolution formed by rotating a curve (a flattened parabola) having a locus of points ofapproximately about its axis of symmetry, wherefis the focal length of the parabola, x and y are Cartesian coordinates, and A is the radius of ring focus C. It should be clear that the symmetrical displacement or offset of the parabola for the main reflector may be somewhat greater than A since none of the rays directed to the main reflector via the subreflector are precisely parallel to the system axis. The significant consideration here is that the main reflector possess a configuration in the form of a figure of revolution having the ring focus C as its prime focus.

The other frequency of RF signal is supplied by feed 19 which directly illuminates main reflector 11. Because of the presence of the flat surface of reflector 11 in the region A about its vertex, and the fact that feed 19 has as its phase center a point rather than a ring, some phase error will occur with consequent degradation of efficiency of'feed 19. However, this effect is relatively insignificant where, as here and as is typical of the Gregorian antenna, the high performance feed is positioned at the virtual focus rather than the prime focus.

Accordingly, rather than having a substantial portion of the RF energy transmitted by feed 12 lost in the form of scattered or spillover radiation as a consequence of the mere presence of the second feed, the present invention teaches that the second feed may be appropriately shaped to redirect that RF energy onto the surface of the main reflector through a ring focus. The result is a major improvement in the efficiency of the antenna system, with any aperture blockage attributable to the presence of the subreflector l8 alone, virtually independent of the existence of the second feed located at or adjacent this subreflector.

While the present invention has been disclosed and illustrated in terms of a specific preferred embodiment, it will be appreciated by those skilled in the art to which the invention pertains that variations of these specific details of construction which have been illustrated and described may be resorted to without departing from the spirit and scope of the invention. For example, while the preferred embodiment has been described from the standpoint of a transmitting antenna it will be understood that it may be used as a receiving antenna also. Accordingly, limitations on the invention are to be imposed only as required by the appended claims.

What is claimed is:

' l. A Gregorian antenna system, comprising a main reflector,

a subreflector,

a first feed for illuminating said main reflector via said subreflector, and

a second feed for directly illuminating said main reflector,

said second feed being shaped, and positioned relative to said subreflector, to produce a ring focus for energy reflected from said subreflector toward said main reflector,

said main reflector being shaped to have said ring focus as .its prime focus.

2. The antenna system according to claim 1, wherein said second feed has a cylindrical external surface with an axis coincident with the axis of said subreflector.

3. The antenna system according to claim 2, wherein said ring focus has a center lying on said axis and a radius A, and said main reflector is a surface of revolution formed by rotation of a curve having a locus of points (yA) =4fx about its axis of symmetry, wherefis the focal length of the curve, and x and y are Cartesian coordinates. 4

4. A Gregorian antenna system, comprising a main reflector,

a subreflector positioned relative to said main reflector to share a common prime focus therewith,

first feed means for illuminating said subreflector,

and

second feed means for illuminating said main reflector and for creating a ring focus for transfer of energy between said subreflector and said main reflector,

said main reflector having a reflecting surface shape with a prime focus coincident with said ring focus.

5. The antenna system according to claim 4, wherein said second feed means includes a feed and a symmetrical reflective external surface for said feed.

6. The antenna system according to claim 5, wherein said symmetrical reflective external surface is cylindrical in shape and shares a common axis with said subreflector.

7. The antenna system according to claim 5, wherein said ring focus has a radius A about the axis of said antenna system, and

said main reflector is a figure of revolution defined by rotating about its axis of symmetry a curve having a locus of points according to (y-A) =4 fx wherefis the focal length of'the curve, and x and y are Cartesian coordinates. 8. A Gregorian antenna system, comprising a main reflector, a subreflector for reflecting energy toward and receiving energy from said main reflector, first feed means for illuminatingsaid main reflector,

and second feed means for illuminating said subreflector, said first feed means being positioned relative to said subreflector and said main reflector to intercept substantially all of the energy emanating from one of said subreflector and said main reflector, and having a surface covering and shape to reflect the incident energy substantially completely onto the surface of the other of said subreflector and said main reflector. 9. The antenna system according to claim 8, wherein Said main reflector is shaped to possess a prime focus conforming to the focus associated with energy reflected from said first feed means. 

1. A Gregorian antenna system, comprising a main reflector, a subreflector, a first feed for illuminating said main reflector via said subreflector, and a second feed for directly illuminating said main reflector, said second feed being shaped, and positioned relative to said subreflector, to produce a ring focus for energy reflected from said subreflector toward said main reflector, said main reflector being shaped to have said ring focus as its prime focus.
 2. The antenna system according to claim 1, wherein said second feed has a cylindrical external surface with an axis coincident with the axis of said subreflector.
 3. The antenna system according to claim 2, wherein said ring focus has a center lying on said axis and a radius Delta , and said main reflector is a surface of revolution formed by rotation of a curve having a locus of points (y- Delta )2 4 fx about its axis of symmetry, where f is the focal length of the curve, and x and y are Cartesian coordinates.
 4. A Gregorian antenna system, comprising a main reflector, a subreflector positioned relative to said main reflector to share a common prime focus therewith, first feed means for illuminating said subreflector, and second feed means for illuminating said main reflector and for creating a ring focus for transfer of energy between said subreflector and said main reflector, said main reflector having a reflecting surface shape with a prime focus coincident with said ring focus.
 5. The antenna system according to claim 4, wherein said second feed means includes a feed and a symmetrical reflective external surface for said feed.
 6. The antenna system according to claim 5, wherein said symmetrical reflective external surface is cylindrical in shape and shares a common axis with said subreflector.
 7. The antenna system according to claim 5, wherein said ring focus has a radius Delta about the axis of said antenna system, and said main reflector is a figure of revolution defined by rotating about its axis of symmetry a curve having a locus of points according to (y- Delta )2 4 fx where f is the focal length of the curve, and x and y are Cartesian coordinates.
 8. A Gregorian antenna system, comprising a main reflector, a subreflector for reflecting energy toward and receiving energy from said main reflector, first feed means for illuminating said main reflector, and second feed means for illuminating said subreflector, said first feed means being positioned relative to said subreflector and said main reflector to intercept substantially all of the energy emanating from one of said subrefLector and said main reflector, and having a surface covering and shape to reflect the incident energy substantially completely onto the surface of the other of said subreflector and said main reflector.
 9. The antenna system according to claim 8, wherein said main reflector is shaped to possess a prime focus conforming to the focus associated with energy reflected from said first feed means. 